On RecA protein-mediated homologous alignment of two DNA molecules. Three strands versus four strands.

The recA protein from Escherichia coli can homologously align two duplex DNA molecules; however, this interaction is much less efficient than the alignment of a single strand and a duplex. Three strand paranemic joints are readily detected. In contrast, duplex-duplex pairing is detected only when the incoming (second) duplex is negatively supercoiled, and even here the pairing is inefficient. The recA protein-promoted four strand exchange reaction is initiated in a three strand region, with efficiency increasing with the length of potential three strand pairing available for initiation. This indicates that a paranemic joint involving three DNA strands may be an important intermediate in all recA protein-mediated DNA strand exchange reactions and that the presence of three strands rather than four is a fundamental structural parameter of paranemic joints.